Three months ago, the head coach of a professional football team made a terrible, terrible decision. On Sept. 30, up by a point with less than two minutes to go, Carolina's Ron Rivera decided to punt the ball away to the Atlanta Falcons, even though it was fourth-and-1, even though his team was on Atlanta's 45-yard line, even though he had Cam Newton. According to one post-game analysis, Rivera's punt cut his team's chances of winning by at least one-third. "This is it," wrote Aaron Schatz in this space. "This play. Cam Newton is now our poster child for bad fourth-down decision-making."

Two days ago, a different head coach of a different professional football team made a different sort of decision. On Jan. 6, up by two touchdowns in the first quarter, Washington's Mike Shanahan decided to leave his rookie quarterback in the game, even though Robert Griffin III had tweaked his knee, even though Griffin had strained that knee the month before, even though Griffin had torn two ligaments in that knee in college. The Redskins lost, and Shanahan's quarterback may have two new partial tears in his knee ligaments. "Whatever reputation Shanahan had is forever scarred by this," wrote Dave Kindred on Twitter. "FIRE SHANAHAN. NOW."

But these scenarios are not the same. In one, we can say exactly why the coach was wrong. We can run the numbers to produce an estimate-not a perfect one, but a pretty good one-of Rivera's wrong-headedness. We can express his failure as a change in betting odds. In the other, there is no way to know if Shanahan was wrong or right. We can wring our hands and call him names, but we have no math to fortify our hindsight. We have only the crude and shaky logic of what happened.

The argument contra Shanahan takes two forms that have been mixed together in a soup of sports-fan sanctimony. First, we hear the coach has failed in terms of football: Griffin looked bad out there (against the fourth-ranked defense in the league), so Shanahan should have pulled him sooner; that would have given Washington a better chance to win, even in the here and now. Second, we're told the coach has failed in terms of ethics: By refusing to remove his starter, he put both Griffin and the team at greater risk; it was, as Josh Levin implied on Sunday, a case of "medical malpractice."

Let's start with issue one: Did Shanahan, like Rivera, screw his odds of winning this one specific game? That's a fine topic for sports-talk radio, but not so good for rational analysis. The backup (who happens to remind his coach of Drew Brees) might have had a better game than Griffin, but he didn't play so well when he did come in, and his success in prior weeks came against some lousy teams. If it weren't for the injury, Shanahan's decision to leave the backup on the bench would have been debated but not deplored.

What about issue two, and the charge that Shanahan risked his star's career? Was it foolish—or unethical—to leave Griffin in the game? This is where a lack of data can be distorted into pseudoscience. "I did put myself at more risk being out there," the quarterback said on Sunday night. This statement went unchallenged in the media, even as we were told that players don't understand what's good for them. (Earlier in the day, when Griffin said he felt well enough to play, he was not to be trusted.) We know it's dangerous to play football on a tender knee. That's common sense.

But common sense isn't always so reliable. The treatment of a knee injury is tricky and subjective, David Brown reminds us in a thoughtful piece for the Washington Post. ("There's no cookbook," as orthopedist-to-the-superstars James Andrews likes to say.) One doctor explains to Brown that a college injury like Griffin's doesn't make a player more likely to bust his knee again. Neither does the sort of sprain that Griffin suffered in December predispose an athlete to a tear in his ACL. Griffin may have been ineffective, but he wasn't necessarily fragile-in which case Shanahan's only folly was to think his ailing star could win the game.

On the other hand, Griffin may have been at greater risk of re-injuring his knee, the doctors say, because a painful knee would change the way he moved. How much greater risk, exactly? It's hard to say. And if Griffin did get hurt, how serious would it be? It's hard to say. These questions are not just murky but opaque. Doctors have no clever algorithms for determining how well or poorly Shanahan played the numbers; there is no Moneyball for joint injury. In place of data, sports teams have gurus like Dr. Andrews, who (in the course of hawking his new book) told USA Today, "I've been a nervous wreck letting [Griffin] come back as quick as he has … he's still recovering and I'm holding my breath."

Andrews surely knows as much about the knee as any other living soul, but his nerves are not dispositive. He described having the very same reaction to the Minnesota Vikings' Adrian Peterson, who tore two ligaments in December 2011 and this year led the NFL in rushing. "I was a nervous wreck watching him play game after game," Andrews said of Peterson. "Every time he'd get tackled I'd shudder."

(What about Andrews' statement that he never got a chance to look at Griffin's knee when the player injured it in December? He now confirms Shanahan's version of events: "Shanahan looks at me like, ‘Is he OK?' and I give him the ‘Hi' sign as in, ‘He's running around, so I guess he's OK.'")

When it comes to knee injuries, experts can't agree even on some basic principles. After Derrick Rose tore a ligament last season, a University of Chicago surgeon who does scores of ACL reconstructions every year argued it was because the Bulls point guard had missed so many games with other injuries—"if he was a little bit stronger, his muscles would have fired and kept his knee still." But David Altchek, one of the best-regarded sports doctors around, said that was hogwash. "There is no evidence that wear and tear, or that kind of issue, playing too much, really has any correlation with ACL injuries in any sport that we've ever studied," Altchek told the AP.

In the face of all this uncertainty, there certainly was a case for leaving Griffin in the game. He may have given the Redskins their best chance of winning, and if he did strain his ACL or suffer a low-grade tear he'd have a full offseason to recover. Of course there was the possibility of a worse outcome, too, that Griffin would hurt himself more severely and need another surgery. In thinking that one through, Shanahan could have drawn on some statistics: 63 percent of pro football players who undergo a primary ACL reconstruction return to play, an average of 10 months later. Higher draft picks like Griffin tend to have better odds than most.

So Shanahan did what he thought was best for his team. (In this case, at least, a head coach and his player would seem to have the same incentives: They both want to win the game, and they both want Griffin to have a long and prosperous career.) He did what people with his job must do: He weighed the risks and benefits. When it comes to punting on fourth down, we can tell if a coach is playing smart or playing stupid. When it comes to on-field injury, the probabilities are harder to grasp. Is Shanahan to blame for what happened Sunday evening? Has he made a terrible mistake? There's no way to know for sure. You'll have to trust your gut.

Daniel Engber is a senior editor at Slate, and writes for the magazine about science, culture, and sports.